Electron discharge device circuit



Aug. 8, 1939 W. VAN B. ROBERTS ELECTRON DISCHARGE DEVICE CIRCUIT FiledOct. 7, 1955 2 Sheets-Sheet l INVENTOR WALTER VAN B. ROBERTS ATTORN EY Iw. VAN B. ROBERTS 2,168,782

ELECTRON DISCHARGE DEVICE CIRCUIT Aug. 8, 1939 Filed Oct. 7, 1955 2Sheets-Sheet 2 -OOOOOOOOOOO mom/er 7 Rf? AMI.

REC. I REC.

MN REFZEC'T/l/i lNVENTOR WALTER VAN B.ROBERTS RES/.5 771N655 I BY M-WATTO R N EY Patented Aug. 8, 1939 UNITED STATES OFFIC Walter van B.Roberts, Princeton, -N. 5., assignor to Radio Corporation oi erica, acorporation of Delaware Application October 7, 1935, Serial No. 43,880

7 Claims.

This invention relates to electron discharge device circuits, and moreespecially concerns such circuits which are adapted for use withelectric cables, particularly the high frequency coaxial conductor typeof cable.

In the past, in using long cables for the transmission of messages, ithas been customary to employ repeaters at predetermined intervals alongthe cable to amplify the message waves to overcome losses due toattenuation, leakage, etc.

These repeaters, which may be either of the electhe foregoingdisadvantages and has for one of its objects to provide a moreeconomical installation for amplifying a wide band of frequencies. Afurther object is to provide an amplifier which is adapted to beactually inserted in a transmission line for overcoming the energylosses of the line. A still further object is to provide a transmissionline having a negative damping characteristic for the production ofsustained oscillations of a wave length determined by the length of theline.

An important advantage of the present invention is that in case offailure of an amplifier constructed in accordance with the principlesthereof, signals are nevertheless transmitted therethrough withoutappreciable loss, although of course without the gain characteristic ofthe amplifier in its normal condition.

In brief, the invention comprises an expander line" (exhibiting anegative conductance) which is adapted to be actually inserted in and asa portion of the transmission line. According to one embodiment of theinvention, there is employed as an amplifier an expander line sectioncomprised of a network of series impedances having connected thereacrossa pair of serially connected electron discharge devices providingnegative conductance properties. This section is constructed to have acharacteristic impedance to match the line in which it is inserted.Accord- The present invention is designed to overcome ing to another andpreferred embodiment, the amplifier is an electron discharge devicehaving elongated electrodes, possessing distributed negative conductancecharacteristics and functioning by dynatron action, which is made to actas a concentric transmission line. A third embodiment of the inventionis a double cold cathode device exhibiting negative resistance effects.A- fourth embodiment illustrates the manner in which any of the firstthree embodiments can be used in a receiving. circuit.

Other objects, advantages and features will appear from a reading of thefollowing detailed description which is accompanied by drawings,wherein:

Figs. 1 and 2 show, by way of explanation only," well known types ofsections of smooth and lumped transmission lines, respectively;

Fig. 3 illustrates a section of expanding line for amplifying signals,in accordance with one embodiment of the present invention;

Fig. 4 illustrates a cross-section of a portion of a transmission lineof the two conductor or coaxial type, equipped with an expanding linesection, in accordance with another embodiment of the present inventionfor obtaining distributed negative conductance;

Fig. 4a is a transverse cross-sectional view of the expanding linesection of Fig. 4, along the lines in-ta;

Figs. 5 and 6 show cross-sectional views of other expanding linesections, in accordance with the principles of the present invention,for obtaining distributed negative conductance along the section; and

Fig. 7 discloses a centralized radio receiving system containing anexpanding line section, in accordance with the invention.

Fig, 1 illustrates an ordinary well known type of transmission line, forexample, a two-wire line adapted to transmit a wide band of frequencies.In such a line the series inductance and resistance, and the shuntcapacity and shunt or leakage conductance are uniformly distributedalong the line. The damping coeflicient of such a line is given by theequation:

1 c l I Tiara zwhere designed to have a characteristic impedance equalto the characteristic impedance of the smooth line, then such sectionsmay be inserted in the smooth line without reflection losses and. willact substantially like the smooth line for the above mentionedfrequencies commonly referred to as the frequencies below cut-jolt ofthe section shown in Fig. 2.

It will be noted from Equation 1 that k is made negative andsuiliciently large in value, the damping coeiilcient given by thisequation will become negative. and signals passing through the line willincrease in energy as they go along rather than decrease as they do inthe ordinary dissipative type of line.

Fig. 3 shows how, in accordance with one embodiment of the invention, anegative conductance may be shunted across the capacity of the sectionshown in Fig. 2. In Fig. 3 a pair of tubes '1, '1 are connected so thatthe excitation of the grid G of each tube is opposite in phase to thepotential on the plate P of the tube, thus resulting in a negativeconductance across the line or a magnitude given approximately by theformula:

where a is the amplification constant and R9 the internal resistance ofthe tube. This equation may be readily derived as follows: If the gridof a tube is excited in opposite phase to its plate and with equalvoltage, and the amplification constant a is large compared to unity,the plate circuit will exhibit negative conductance substantially equalto the mutual conductance of the tube. Since two such tubes areconnected in series across the line if Fig. 3, the negative conductancewill obviously be one-half of this value. From Equation 1 it may be seenthat an expension of the signals passing through the section Fig. 3,will occur when the mutual conductance of the individual tube is morethan twice the series resistance of the section divided by the square ofthe characteristic impedance of the section or line; in other words, ifthe characteristic impedance of the line then negative damping occurs ifModern amplifier tubes have mutual conductance far greater thannecessary to produce this expanding action. (Hence it is preferable touse very small tubes with small capacities and exciting currents, anddesigned for long life.) Therefore, if a section as shown in Fig. 3 andconstructed with a characteristic impedance to match a given line isinserted in the line, it will make up for the loss in a considerablelength of the line. Thus, by inserting such sections at intervals alonga transmission line, the signals travelling along it may be preventedfrom becoming weaker at the receiving end. The shunt capacity of Fig. 3may, if desired, consist solely of the tube capacities.

An important advantage of this method of amplifying signals ascontrasted with the ordinary known type of vacuum tube repeater forinserting gain in a long line isthat whereas the ordinary repeatercompletely interrupts the transmission of signals when a repeater falls,5

for example due to tube failure, the expander section, Fig. 3, inaccordance with the invention, does not interrupt the passage of signalsin case of tube failure but allows the signals to continue to passtherethrough exactly as they traverse any portion of the ordinary line.Hence, if each expending section is designed for only a small amount ofgain, failure of one section will merely cause the signals arriving atthe line terminals to be a small amount weaker than normal, which amountcan be made up by a little additional gain at the terminals of the line.

In Fig. 3 resistance r" and capacity 0" provide grid bias in the usualfashion in connection with grid leaks r. Condensers c are blocking 2condensers, and chokes z are provided to allow energization of the tubeswith the necessary direct current from battery B. One direct currentenergization connection to the line is all that is necessary for all theexpanding sections, provided the line is otherwise insulated fromground.

For very high frequency signals it would be necessary to useimpractically small tubes and numbers of sections ii the method of Fig.3 were to be attempted, and consequently for such ultra highfrequencies, the arrangement of Fig. 4 is preferred.

Fig. 4 shows to the left and right of the vertical dotted lines V, V andV, V, respectively, sections of what is commonly called a co-axialconcentric transmission line consisting of a circular outer conductor 0with a circular inner condoctor I coaxial therewith. For a more detaileddescription of various types of coaxial lines, reference is herein madeto United States Patent No, 1,918,418, granted to H. W. Dudley, October30, 1934.

Between the vertical dotted lines, is the preferred form of amplifier ofthe invention, comprising a portion of a geometrically similar con- 4'centric transmission line arranged to have distributed negativeconductance between an inner conductor S and the coaxial outer conductorA. This arrangement comprises an electron discharge device having anevacuated glass envelope E containing within it an outer conductor A, aninner co-axial conductor S in the form of a grid, and in indirectlyheated cathode C. If desired, part of the envelope E may be dispensedwith and the outer conductor A used as part'of 5 the envelope. ConductorA may be made to be an extension of the outer conductor 0 of thedissipative line, and conductor S an extension of I, Fig. 4. except thatinner conductor S is made of a bundle of conductors parallel to the axisof the structure or may be formed as a cylinder of wire mesh. In otherwords, conductor A may be made to have, if desired, the same diameter as0 and conductor S the same diameter as I. Within the inner conductor Slies an indirectly heated 6 cathode C. Conductor S is maintained at ahigh average positive potential by connection through a choke to abattery B while outer conductor A is maintained at a lesser averagepositive potential,

Thus, by means of secondary emission of elec- 7 trons from outerconductor A there is made to appear between conductors S and A anegative conductance. This eiIect, as well as suitable values of voltagefor producing it, is well known in ordinary tube structures as dynatronaction. Ac- 7 cording to the present invention, however, the system ofFig. 4 is made to act as a concentric transmission line and the negativeconductanceo distributed along it produces an expanding ac-- tion uponsignals travelling therealong. Its action is identical with thatdescribed in connection with Fig. 3 except that in this case, sincethere are no lumped constants, there is no cutoff frequency until thefrequency becomes so high that electron time of flight interferes withthe production of sufficient negative conductance to provide therequired expanding action. In practice, it is preferred to provide alarge capacity indicated at K but distributed all along between cathodeC and inner conductor S so as to assure that no large radio frequencydifferences of potential exist between them. A single set of batteries Bmay be, employed for polarizing the conductors of the coaxial line. Itis only necessary that the characteristic impedance of the expandersection shall be the same. as that of the rest of the line. Since thecharacteristic impedance depends only upon the ratio of the innerdiameter of the outer conductor A to the outer diameter of the innerconductor S, it is obvious that the expander section may be madephysically similar to or larger or smaller, according to convenience,than the rest of the line and yet be inserted in the line withoutreflection of signals as they pass from the expander sectionto theordinary line, or vice versa. In other words, expander sections of thesort shown in Fig. 4, between the vertical dotted lines, do not need tobe made identical in dimension with the coaxial transmission line intowhich they are inserted.

As mentioned in connection with Fig. 3, the negative conductanceavailable from dynatron action in ordinary tube structures is greatly inexcess of the value required to produce expansion characteristics. Thusa relatively short section of expander line will make up for the lossesof a much longer section of ordinary line. On the other hand, if inpractice it is found preferable to utilize a cathode having far lessemission than ordinarily provided in vacuum tubes, the entire line maybe constructed of the expander type, but with the emission so small thatthe negative conductance effect merely balances out the naturaldissipation of the line.

Fig. 4a shows a cross-section of the expander line of Fig. 4 along theline 4a--4a.

, Fig. 5 shows a cross section of a balanced transmission line built inthe form of a pair of elongated vacuum tube structures acting inprinciple like Fig. 3 but providing a distributed negative conductancealong the line. The line proper consists of two plates or strips ofconductor A, A which also act as anodes cooperating with a centralcathode C, preferably of the indirectly heated type. Capacities K areprovided, preferably in continuous manner, but in any case at frequentintervals along the length of A, A compared to the wave length employed,between the grid structures G controlling the current to one side of theline, and the other side of the line, in order to energize the grid G atevery point in opposite phase from the potential of its cooperatingadjacent plate A at the corresponding point. The continuous capacity Kmay take the form of closely adjacent'plates extending throughout thelength of the anodes and grids. Grid leaks GL, GL' are also provided,and a cathode resistor or bias battery may be employed for bias. Thedirect current connections are only required at one point in theexpander line, which may be anywhere along the line. The two plates orline conductors A, A are maintained at an average positive potential bya battery B, B through choke coils L, L'. In this case the negativeconductance per unit length of line is again equal to approximatelyone-half the mutual conductance per unit length of each tube structure.The section of line in accordance with the cross section of Fig. 5 maybe used interchangeably with the type of section shown in Fig. 3 butwith the added advantage that, like the structure of Fig. 4, it has noinherent cut-off frequency. In Fig. 5 a pair of grounded strips P arerun parallel to the oathode to reduce the control action of one gridupon the other plate. These strips, if desired, may be connected atfrequent intervals to the cathode. A screen structure at positivepotential could, if desired, be employed for the same purpose, orseparate cathodes could be used with screening or space between them toprevent cross control action of one grid on the plate current from thecathode of the other tube structure. The cathode c, of course, must beconnected to ground at at least one point.

The methods of connecting a symmetrical or balancedline to anunsymmetrical line such as the concentric co-axial type, are well knownand need not be discussed here, since they form no part of the presentinvention per se.

Fig. 6 shows a cross section of an elongated structure designed tooperate as a two-wire transmission line. This structure consists of anevacuated envelope E containing within it two pairs of oppositelydisposed metallic strips or electrodes D, D and F, F, the envelope beingimmersed in an electromagnetic field produced by coil M. This device maybe given a negative conductance characteristic between the two sides ofthe line D, D by coating conductors D, D with electron emissive,material so as to produce secondary emission from impact by primaryelectrons, applying a high positive potential to the collector strips F,F from a battery B, and connecting the two sides of the line D, D in themanner shown in the drawmgs.

The operation of the system of Fig. 6 is as follows: An electron fromone conductor D, which applicant chooses to call a dynode, will beattracted toward the other dynode D with velocity increasing at firstdue to the accelerating influence of positively charged collector stripsor anodes F, F, and then decreasing after passing the central positionof strips F, F due now to their decelerating influence, until it strikesthe other dynode, whereupon it will splash out other electrons and theywill be attracted back to the first dynode and thus continue tooscillate back and forth with a frequency determined by the distancebetween the dynodes D, D and the voltage of battery B, until theyultimately drift over to the collector strips F, F. A strong magneticfleld extending between the two dynodes caused by coil M surroundingenvelope E, and acting parallel to the motion of the electron, will tendto prolong these oscillations and prevent the electrons reaching thecollector strips F, F until after several back and forth trips betweenthe dynodes.

In the foregoing, it has been assumed that an alternating voltage isimpressed between dynodes from an outside source 0, as shown, and if thefrequency of this voltage iii substantially the same as the frequency ofelectron oscillation, then starting from a few lectrons, such as arealways emitted by photoel tric action from the dynodes, the number ofelectrons oscillating within the tube will be built up by secondaryemission from the dynodes produced by the applied alternating voltageuntil a considerable average current flows through milliammeter I. Ithas been determined experimentally that as" the voltage B is increased,there will be a value of voltage where further increase in voltagecauses a decrease of current through I. This decrease constitutes anegative resistance with respect to the points to which the battery'isconnected. This negative conductance is the sum of the negativeconductance between the collectors F, F and the two dynodes D, D, sothat there exists between the two dynodes themselves a negativeconductance equal to one-quarter of the negative conductance presentedto the battery. This is because two conductances arranged in series haveone-quarter the value of the two conductances in parallel. Thus far wehave considered only a short length of structure of the cross sectionshown in Fig. 6, which together with its method of operation is known inthe art.

In accordance'with the present invention, the structure whose crosssection is shown in Fig. 8 is elongated to any convenient length and isutilized as a portion of a transmission line just as described withreference to Fig. 4,.or Fig. 5, since it has the property of presentinga negative conductance between the two sides of the line, whichconductance is uniformly distributed over the full length of the line.In Fig. 6, small bypass condensers c' are provided to permit the ultrahigh frequency exciting voltage to reach the dynodes but which are toosmall to form an easy path for the relatively low frequency currents tobe transmitted along the line. These condensers are shunted by chokecoils X to provide a direct current path. The batteryand high frequency.connections need only be made to one point on the expander line as boththe direct voltage and the exciting voltage travel all over the line.The phase of the high frequency exciting voltage is immaterial for theproduction of the negative conductance, and, therefore, this negativeconductance will be produced throughout the entire length of thestructure. The advantage of the arrangement shown in Fig. 6 is that itis unnecessary to provide any source of initial electron emission suchas a hot cathode, although it would be preferable to treat the surfacesof the dynodes D, D with a radio-active material or otherwise provide areliable source of a few electrons for starting the action which resultsin the negative conductance. Just as in the case of all the otheramplifying arrangements here disclosed, the advantage is retained thatin the case of failure of the negative conductance the signals continue,nevertheless, to pass along the expander line without appreciablediminution, although, of

While the invention has its primary applies-- tion to long lines fortransmitting a wide band of frequencies not exceeding low radiofrequencies such as the modulation frequencies of television programs,the method of amplification disclosed my be employed for many otherpurposes. For example, if an arbitrary length of expander line isterminated reflectively, waves originating from random infinitesimaldisturbances will travel back and: forth over it with increasingamplitude, thus setting up sustained oscillations of a wave lengthdetermined by the length of line employed. As an illustration, if theline is open at both ends, the wave length will be twice the length ofthe line. By utilizing a large structure adapted to have very smallinherent losses, the frequency stability of such an oscillator may bemade very great and the wave length generated will not be altered by theconnecting of additional tube capacities across the line as is the casein present practice. Such an arrangement is particularly advantageouswhere frequencies to be generated are so high that the capacity effectsof ordinary tubes attached to ordinary resonant lines seriously affectsthe generated frequency.

What is claimed is:

l. A transmission line section comprising two terminals each having inseries therewith con centrated impedances, and electronic means forproducing negative shunt conductance across said terminals, said meansincluding a pair of electron discharge devices each comprised of anode,cathode and grid electrodes, the anode of each device being directlycoupled to one terminal and capacitively coupled to the grid of theassociated device and to the other anode, the cathode of said devicesbeing connected together, means for exciting the grid of each device inopposite phase to the potential of its cooperating anode, the mutualconductance of each device being greater than twice the seriesresistance of the line section divided by the square of thecharacteristic impedance of the line section.

2. The combination with a plurality of serially arranged sections oftransmission line each having substantially uniformly distributed seriesinductance and resistance and shunt capacity at the frequency ofoperation, of a pair of electron discharge devices connected in seriesacross each section of line, means for exciting the grid of each tube inopposite phase and with substantially equal voltage to the anodethereof, whereby each tube exhibits negative conductance, the magnitudeof negative conductance of each tube being greatenthan twice the seriesresistance of the associated section of line divided by the square ofthe characteristic impedance of the section of line.

3. The combination with a section of transmission line havingsubstantially uniformly discompared to unity and including a grid and ananode, means for exciting the grid of each tube in opposite phase andwith substantially equal voltage to the anode thereof, whereby each tubeexhibits negative conductance substantially equal to the mutualconductance of the tube, the magnitude of the mutual conductance of eachtube being greater than twice the series resistance of the section ofline divided by the square of the characteristic impedance of thesection of line.

4. The combination with a plurality of serially arranged sections oftransmission line each having substantially uniformly distributed seriesinductance and resistance and shunt capacity at the frequency ofoperation, of a pair of electron discharge devices connected in seriesacross each section of line, means for exciting the grid of I each tubein opposite phase and with substantially equal voltage to the anodethereof, whereby each tube exhibits negative conductance, the magnitudeof negative conductance of each tube being greater than twice the seriesresistance of the associated section of line divided by the square ofthe characteristic impedance of the section" of line, said sections eachbeing designed to have a characteristic impedance which matches theimpedance of the line into which it is inserted.-

5. A transmission line section comprising two terminals each having inseries therewith concentrated impedances, and electronic means forproducing negative shunt conductance across said terminals, said meansincluding a pair of electron discharge devices each comprised of 6. Aline section adapted to be serially inserted in a two conductortransmission line, said section being only a fraction of the wavelengthat the highest operating frequency and having distributed seriesinductance and resistance and negative shunt conductance in the form ofelectronic means, said electronic means comprising a pair of electronspace paths in series, said paths, when electrically conductive, havingsufflcient negative shunt conductance to oflset at least a portion ofthe damping of said line, said section having substantially the samecharacteristic impedance as said transmission line when said electronicmeans is not electrically conductive, the magnitude of negativeconductance per unit length of each space path being approximatelyone-half the mutual conductance per unit length of line section.

7. A balanced transmission line section having' negative attenuation andadapted to be inserted in a dissipative transmission line to ofiset apart of the attenuation thereof, comprising series and shunt impedanceelements arranged to provide a passive transmission line section ofpredetermined characteristic impedance, a cathode element symmetricallydisposed between both sides of said balanced line section, means forcausing a flow of electrons from said cathode to each of said sides,agrid in each of the last-named electron flow paths for controlling themagnitude of the electron flow therein, and an alternating currentconnection from each of said grids to that side of the line sectionwhose electron current is controlled by, the other of said grids, themutual conductance between each of said grids and that side of the linesection whose current is thereby controlled being sufllcient to renderthe resultant damping of said section negative, the total mutualconductance in said section between each oi said grids and that side ofthe line which it controls being greater than twice the total seriesresistance of said section divided by the square 01' the characteristicimpedance of said section.

WALTER VAN B. ROBERTS.

